As a compressor used in a refrigerating circuit of an air conditioner, there is known a conventional centrifugal compressor as disclosed in Japanese Patent Application Laid-Open Gazette No. 5-340386.
The above conventional centrifugal compressor will be schematically described below. As shown in FIG. 6, in a casing (a), a motor room (b) and an impeller room (c) are formed. The motor room (b) contains a motor (d) and the impeller room (c) contains an impeller (rotary vanes) (f) directly connected to a driving shaft (e) of the motor (d). Further, the casing (a) is connected to an inlet pipe (g) as opposed to a center portion of the impeller (f) and is connected to a discharge pipe (h) as opposed to an outer peripheral portion of the impeller (f).
When the motor (d) is operated to rotate the impeller (f), a fluid is sucked into the impeller room (c) through the inlet pipe (g), is subjected to centrifugal force into a radially outward flow, is compressed, and is then discharged through the discharge pipe (h).
Upper and lower ends of the driving shaft (e) are inserted into respective through holes (i1, i1) of bearing plates (i, i) fixed on the inner surface of the casing (a). Further, the outer periphery of the driving shaft (e) has herringbone grooves (e1, e1) formed at respective positions opposed to the inner peripheries of the through holes (i1, i1). The herringbone grooves (e1, e1) forms dynamic pressure gas bearings between the driving shaft (e) and the bearing plates (i, i).
In detail, the rotation of the driving shaft (e) produces respective gas layers due to gas pressure between the driving shaft (e) and the inner peripheries of the through holes (i1, i1) so that the driving shaft (e) is rotatably supported by the gas layers in non-contact with the bearing plates (i, i).
The dynamic pressure gas bearing of this kind produces a gas layer only by a rotation of the driving shaft (e) in a single direction to rotatably support the driving shaft (e). Therefore, the dynamic pressure gas bearing functions as a bearing only when the driving shaft (e) rotates in a rotational direction of the impeller (f) under operation of fluid compression of the compressor.